Even if both MK2 and MK3 were monocoque, the MK3 is totally different, all parts were redesigned, main frame, fork, rear arm.

Some obvious differences, below pictures of the old MK2:
- the shape of the rear swing arm:


- the shape of the front fork:


- the headset (semi integrated on the MK3):


For the max tire size with 20"wheels, the first limitation comes from the front fork, there is more space at the rear, and it depend if you want a front mudguard or not.

Without mudguard, something like 32mm is the maximum.

With mudguards, 32mm won't fit. 28mm is I think the maximum (I tried with with the original front mud guard and a wheel with Schwalbe Pro One 28x406 and it didn't fit but the Pro-one are actually wider+higher than 28mm, about 31mm on my rims).

Note that as always, it also depends of the width of the rims, wider rims will make the tire wider and flatter what will help.

Pacific Cycles deliver the Birdy R20 11SP with Panaracer Minits (Lite in the pictures, Tough in the specs ???) that are officially 32x406 but I have Minits Lite PT tires on another bike and they are actually only 30mm wide.